<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <style>
        .float {
            height: 200px;
            width: 200px;
            border: 1px solid black;
            background-color: lightgreen;
            float: left;
        }

        .hellow {
            background-color: lightcoral;
            display: flow-root;
            width: 100%;
        }
    </style>
    <div class="float"></div>
    <div class="hellow">HELLOW</div>
</body>

</html>